China continues to amaze us with its
fossil treasures. The latest is a 165 million year old Jurassic mammal, belonging
to my favourite group, the docodontans. Docodontans are one of the earliest
branches of mammaliaforms, the wider group that includes Mammalia. They are our
cousins, and in the last twenty years they’ve transformed our understanding of
mammal ecological diversity in deep time, because they’ve basically done it all
before. This group includes the first mammal specialist diggers, swimmers and
tree-climbers, occupying these niches long before modern mammals were on the
scene. Their teeth were complex, with a triangular arrangement of cusps and
troughs that later convergently evolved in modern mammals. In other words,
docodontans are the trend-setters of the mammal world.
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| The docodontans were incredibly ecologically diverse in the Jurassic. They include climbers like Agilodocodon, swimmers like Castorocauda, and diggers like Docofossor. Amazing palaeoart by April I Neander, University of Chicago. |
This new genus and species is called Microdocodon gracilis. The name says it
all: this creature was tiny and slender. With an estimated body mass of just
5-9 grams, it’s on a par with the modern shrews, Sorex. Unlike them, it has an elongate face and limbs and a long tail,
suggesting it may have been a competent tree-climber as well as ground-scurryer
(this combination is also known as being scansorial). What’s really stunning
about this wee beastie is something common to many of the spectacular fossils
of China: it is exquisitely preserved with almost every bone in articulation. This
includes bones that are usually too small and fragile to survive fossilisation, such as
the hyoid.
So what is a hyoid? You may know it from crime dramas like CSI; it’s the
bone in the throat that is often damaged when a murder victim has been
strangled. If you locate your larynx (the ‘Adam’s apple’) in your own throat,
then feel upward to the area where your chin meets your neck, you can feel it
in there. It’s not in contact with other bones, but floats there, anchored in place with
muscles and ligaments. The hyoid is where your tongue and the other muscles in the floor of
your mouth attach, and it is intrinsic to swallowing and moving the tongue.
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| Position of the hyoid in humans (red). From Wikipedia |
When you’re looking at mammal skeletons in
museums, the hyoid bone is often conspicuous by its absence – it’s a tricky wee
blighter, and I suppose that it is often lost or too fiddly to make it to the
skeletal mount. However, it is an important and unique part of the skeleton,
inherited by vertebrates from their common ancestor over 375 million years ago.
The hyoid originates from the same embryonic structure that becomes the second gill arch of fishes; those loops
of bone that support the fish breathing apparatus. In mammals it has taken on a
special significance, because it’s thanks to the hyoid that mammals literally
suck.
Mammals use the control provided by the
hyoid to suckle and feed on liquefied food, often processed by their complex
teeth, but also including milk from their mothers. Mammals are not the only
animals that suck of course. There are other vertebrates that use the hyoid to
create a vacuum for feeding. The turtle Chelus fimbriata, or mata mata, for example, has a large hyoid and a pair of
flappy cheeks, which it opens suddenly to draw-in passing fish by suction. As
well as suction, this bone allows for special movement of the tongue: lizards and
snakes carry out their characteristic tongue-flicking thanks to their hyoid
bone and attachments.
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| Embryology reveals the origin of the structures of the face. Source |
Due to its role in swallowing, the hyoid
can provide clues about the kind of food being eaten by the earliest mammals. Microdocodon has a saddle-shaped and
complex hyoid that is the clear predecessor of later mammalian hyoid bones.
This tells us that this animal had a muscular throat with good control of the
ability to swallow. The authors suggest that the very modern structure of Microdocodon’s hyoid is linked to the
ability of early mammals with complex teeth to chew their food until it was
well liquefied. This required a sophisticated and controlled ability to swallow
the food. We also infer from the tooth replacement patterns of the first mammals that
they likely fed milk to their young (they had 'milk teeth' followed by adult teeth), which would also require controlled
ability to suck and swallow. Therefore it is likely that this complex hyoid
structure appeared in the common ancestor of docodontans and the rest of the
mammals, and not in earlier mammal relatives, because the latter didn’t have
such complex food processing abilities.
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| More amazing artwork by April I Neander |
Unlike modern mammals, docodontans still had
their middle ear bones attached to their jaws. In modern mammals these bones
have become the malleus and incus, and help provide our extra-sensitive hearing
capabilities. This new Chinese fossil gives us previously unknown information
about how the hyoid was positioned in ancient mammals – and therefore the larger
patterns of its evolution and function. The only other Jurassic hyoid known belongs to Vilevolodon, a
haramiyidan mammal. Because there is an ongoing disagreement about whether
haramiyidans are an early branch of mammaliaforms, or a much more derived
group belonging to crown Mammalia (that’s a blog for another time), the
evidence from Vilevolodon comes with
baggage. Microdocodon however, is a
safe phylogenetic bet, and so uncontroversially clarifies the structure of this
feature in the earliest mammals.
Such a great fossil, and continued proof that docodontans are among the most exciting group of mammals from the Mesozoic.
References
Lemell, P, Beisser, C, Gumpenberger, M, Snelderwaard, P, Gemel, R, Weisgram, J. 2010. The feeding apparatus of Chelus
fimbriatus (Pleurodira; Chelidae) – adaptation perfected? Amphibia-Reptilia.
31: 97-107.
Luo, Z-X, Meng, Q-J, Grossnickle, DM, Liu, D, Neander, AI, Zhang, YG, Ji, Q. 2017. New evidence fo rmammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem.
Nature 548, 326–329.
Zhou, C-F, Bhullar, B-AS, Neander, AI,
Martin, T, Luo, Z-X. 2019. New Jurassic mammaliaform sheds light on earlyevolution of mammal-like hyoid bones. Science,
365:276-279.
